In many environmental and biological samples, the matrix contains large amounts of alkali and alkaline earth metals as compared to the transition elements and rare earth elements. This concentration difference can be 1,000 to 1,000,000 times greater than the transition elements and rare earth elements of interest. Conventional ion exchange concentration methods for determination of trace and ultra-trace level of transition elements and rare earth elements cannot be used since these methods are typically not selective enough for the specific ions.
Iminodiacetate chelating resin has been used for brine type matrixes and offers a number of advantages. In such procedures, the alkali and alkaline earth metals are separated as a class from the transition elements and rare earth elements. Further, the transition elements and rare earth elements can be readily eluted using mineral acids, which is an acceptable matrix for some analytical techniques. However, the transition elements and rare earth elements may be present at insufficient concentrations to be effectively analyzed. Furthermore, no effective means have been provided of coupling the class separation with a technique for chromatographic separation of the individual transition elements and rare earth elements. In that regard, there are considerable difficulties in such coupling for samples in which the trace metals are present and the concentration too low for chromatography or other methods. Further, direct coupling could not be performed because of the vast differences in the eluants used for removing the transition elements and rare earth elements from the chelator columns and those employed for chromatography.
An established technique for separating transition and rare earth elements as a class from alkali and alkaline earth metals is open column chromatography. It uses a variety of tubes packed with resin with only gravity and atmospheric pressure to force a liquid phase through the resin column. Open column chromatography is most often a preparatory technique to perform a separation where a portion of the effluent is collected in a batch mode to be analyzed later by another method.
This particular open column chromatography uses a specific class of chromatographic fixed-phase resin. (A commercially available analytical grade chelating resin is Chelex-100 currently manufactured by Bio-Rad, the resin was first produced in the mid 1960's by Dow under the name Dow A-1). This chelating resin differed from ion exchange resins by using chelating mechanisms to hold elemental ions. The effectiveness as a chelator is pH-dependent and has a wide range of selectivities (10.sup.11). The active group in the resin is iminodiacetate. The resin was first used in the late 1960's for the collection of transition elements from high salt matrixes such as sea water, but no separation of elements was performed on the resin. Those elements that did not chelate were retained on unused sites on the resin, but some were lost from the column when less resin capacity was used than necessary to concentrate all ions from a sample. The resin was used as a collector of transition elements in solutions high in alkali and alkaline earth elements with residual alkali and alkaline earth elements remaining on residual resin capacity. At that time it was stated that no complete separations of specific elements or ions would be possible using this type of chelating resin.
Riley and Taylor published several papers collecting the trace transition elements in sea water, allowing some of the alkali and alkaline earth elements (most notably: Na, K, Ca, Nd Mg, etc.) to flow through a small column and elute all the ions with mineral acids or base. The elemtns were concentrated and the alkali and alkaline earth elements were reduced sufficiently to aid in the detection of the transition elements by atomic absorption spectroscopy.
A separation was developed in 1978 using this resin that did separate the alkali and alkaline earth elements completely from the retained transition and rare earth elements (1, 2, 3, 4). Reference 2 contains a review of the previous work to that point, as well as a description of this new separation method that used the resin to completely separate classes of elements. This work was demonstrated using open column chromatography on sea water prior to analysis by either graphite furnace atomic absorption, x-ray fluorescence or neutron activation analysis. Sea water is of interest as a very difficult matrix but more importantly, it is the most difficult of many real analytical samples. It contains very high concentrations of alkali and alkaline earth elements that are many times greater than the trace transition and rare earth elements which are of primary interest, 10.sup.8 and 10.sup.6 greater, respectively. Almost all naturally occurring samples have this same type imbalance; high concentrations of alkali and alkaline earth elements compared to the trace transition and rare earth elements. Since it is the alkali and alkaline earth elements that interfere with most analytical chemical instrumental analyses, the ability to preconcentrate the trace transition elements and to totally remove the alkali and alkaline earth elements prior to analysis is a powerful tool for analytical chemistry.
Due to its wide applicability, this procedure has been applied to many acid-digested samples prior to instrumental analysis since 1978. It has been applied as a sample preparation method for the analysis of trace elements in biological, botanical, brines, sea water, fresh water, and other samples (5-11). It was further modified using a new method to directly introduce the resin containing the trace elements after elimination of the alkali and alkaline earth elements into a nuclear reactor to perform neutron activation analysis (6, 8, 10) and other instrumental methods. Several of these applications required modification of the final sample form for compatibility with a particular instrument but most of the basic method for the separation remained unchanged from the original 1978 (4) papers to present.
The concentration of the transition and rare earth ions and subsequent separation of alkali and alkaline earth ions are difficult to control because they are dependent on many parameters. Experienced chromatographers are often unable to use it in a routine manner because of these difficulties. Several examples identify and documented the challenge of controlling the method of this particular open column chelation chromatography. After reviewing literature, four chromatographers could not obtain the optimal efficient recoveries reported for several elements (12). Another group tried to control the trace element preconcentration without the separation by using flow injection and complained of the difficulty of controlling the resin that shrinks and swells in different ion complexed forms and at different pHs necessary to perform the concentration (13). They then analyzed the sample by inductively coupled plasma spectrometry after batch collection. They were unsuccessful in demonstrating quantitative recoveries for many of the trace elements and were not able to identify conditions that would provide quantitative results for the elements tested. All of these papers mentioned used Chelex-100 resin.
One problem with the open column system is that all of the parameters cannot be totally controlled because of the physical constraints of the system. A recent paper describing the inability to achieve complete retention indicates how difficult it is to control this type of reaction under open column conditions, even for experienced chromatographers (12). It is very easy to lose control of the chemistry, or change one parameter that will affect the retention or elution mechanism and cause an error.
The flow rate cannot be controlled in the open column system. To increase the flow rate researchers used larger particle sizes than are recommended in the original paper. This is one common problem introduced by chromatographers when attempting to duplicate the conditions of the original method. If the same particle size resin were used in open column than is necessary for the pressurized system, the flow rate would decrease dramatically and the separation time would increase to approximately eight to ten hours per open column.
The resin that is used in the prior open column methods (Chelex-100) is soft and could be crushed by pressure if used in pressurized columns with dimensions necessary to achieve optimum capacity and at optimum flow rates. Chelex-100 has properties similar to a gel and is not cross-linked sufficiently to permit the resin to function in the pressurized system. It shrinks and swells 50-100% in volume during pH and chelated ion changed.
The open column method does not permit the system to be used directly on-line with an instrument for detection. It is confined to batch mode and prevents the direct coupling of the column to an instrument, to a second column, or to a detector. It also does not permit the addition of the sampl in acid form. This is a new and important procedure for some samples due to the hydrolysis of iron and aluminum, and other elements at the pH range where the resin changes from being a weak ion exchanger to a very powerful chelator (approximately pH 5., (1, 2, 3, 4, 5). This is important to the usefulness of the system for certain types of samples such as biological, botanical, sediment and geological samples that contain large quantities of these elements.